1. Field of the Invention
The invention relates to a method and apparatus for managing irrigation of plants. Irrigation management is based upon the optimum temperate for plant metabolism and an integration of the environment derived from the plant's canopy temperature.
2. Description of the Prior Art
The most common purpose of irrigation is to alleviate crop water stress by the timely application of supplemental water. Current irrigation scheduling methods frequently involve the measurement of soil moisture, atmospheric parameters, and other plant measurements such as canopy temperature, stomatal resistance, leaf color, and leaf water potential. This information is then used with simple decision rules or in more complex decision algorithms that calculate soil water balance, evapotranspiration, or a plant water stress index, in order to estimate the timing and quantity of irrigation application. The procedures require either a considerable effort in time and labor to directly measure soil or plant water status, or the measurement of numerous parameters for use as input to decision-making software for irrigation scheduling. Examples of some of these irrigation scheduling techniques are described by Hearne and Constable (1984, Irrig. Sci., 5:75-94), Villalobos and Fereres (1989, Trans. ASAE, 32(1):181-188), Pleban and Israeli (1989, J. Irrig. and Drainage Engr., 115:557-587), and Rogers and Elliott (1989, Trans ASAE, 32 (5): 1669-1677).
A common theme in the above procedures is that a crop's need for water is assessed indirectly. The development of infrared thermometers has made it possible to measure plant canopy temperature directly. Theoretical and empirical work by Jackson et al. (1981, Water Resour. Res., 17:1133) and Idso et al. (1981, Agric. Meteorol., 24:45), respectively, the contents of each of which are incorporated by reference herein, provide a method for calculating a crop water stress index (CWSI). The CWSI includes a direct measurement of canopy temperature in addition to other environmental parameters. The theoretical definition of CWSI evolves from a description of the energy balance for a plant canopy under non-water stressed conditions. CWSI is a normalized value where 0 and 1 represent completely non-stressed and completely stressed conditions, respectively.
The empirical method of measuring CWSI has been used by Garrot et al. (1990, Irrigation Scheduling Using the Crop Water Stress Index, In: Visions of the Future, Proceedings of the Third National Irrigation Symposium, St. Joseph, MI:ASAE, p. 281-286) to schedule irrigation of cotton, wheat, pecans, and watermellons in Arizona. In these studies, irrigating when CWSI values were between 0.1 and 0.2 produced maximum yields.
Gardner et al. (U.S. Pat. No. 4,876,647, issued Oct. 24, 1989) developed a process and device for determining the the CWSI plants from measurements of air temperature, canopy temperature, relative humidity, and the relative intensity of sunlight.
Recently, the concept of thermal stress in plants has been investigated as a criterion for irrigation management. Mahan et al. (1987, Plant Physiol. Suppl., 83:87) and Burke et al. (1988, Agron. J., 80:553-556) disclosed that optimal enzyme function of a plant is restricted to a range of temperatures, referred to as the thermal kinetic window (TKW). Burke et al. also found that above ground biomass production was positively correlated to the cumulative time that the canopy temperature was within the TKW. Mahan and Upchurch (1988, Envirn. and Exp. Botany, 28:351-357) proposed that plants preferentially maintain a specific temperature, referred to as the normative plant temperature, T.sub.n, in response to this narrow temperature range for optimal enzyme function. Three limitations to this homeothermic behavior were suggested: 1) sufficient energy input to raise the temperature to T.sub.n, 2) sufficient water for transpirational cooling to T.sub.n, and 3) humidity conditions which would allow for transpirational cooling to T.sub.n.
Based on the existence of this normative plant temperature, T.sub.n, and the observed relationship between plant temperate and plant performance, Wanjura et al. (1988, Proceed Beltwide Cotton Production Res. Confs; 183-185) disclosed the use of a threshold canopy temperature to control irrigation of cotton. In subsequent studies, Upchurch et al. (1990, Acta Horticulturae, 278:299-308), Wanjura et al. (1990, Trans. ASAE, 33(2):512-518) and Wanjura et al. (1992, Trans. ASAE, 35 (1):153-159), the contents of each of which are incorporated by reference herein, studied the automatic control of irrigation using continuous canopy temperature measurements. In accordance with these processes, cotton was irrigated whenever the average canopy temperature (measured over a predetermined time period) exceeded a predetermined threshold temperature.